The present invention relates to automatically adjusting the looking window for the detection of ultrasonic signals, by means of follow-up control.
The "looking window" referred to above is to mean, for example, the period during which an ultrasonic receiver is enabled to receive an echo signal. A looking window, therefor, has two meanings, a broad one and a narrow one. Generally, it is just a period during which a particular signal path for electrical signals representing ultrasonic signals is open. The narrow meaning or definition relates to the fact that for and during a specific looking window specific signals are expected to the exclusion of others.
It is customary in ultrasonic testing, for example, of sheet metal or other flat objects, to determine or to predetermine a particular period of time during which an echo pulse must be received so as to discriminate against other ultrasonic signals which could not possibly represent an echo by a defect or flaw in the material. For example, that period of time or looking window must not extend beyond the instant of occurrence of an echo from the rear or back wall, boundary or surface of the object tested. The device which establishes the basis for such a looking window is called a monitor, e.g., a signal monitor, digital monitor, analog monitor, integrating monitor, depending on the type and mode of operation. The looking window is established or generated by such a monitor, for example, by means of test reflections derived from representative test or reference objects, and once established, the looking window remains invariant. By way of example, sheet metal, strips, tubes or pipes may be tested by using test heads which transmit ultrasonic waves transversely to the surfaces of the test objects. For reasons of tolerances of the test objects, as well as of the test equipment, one can test only from the front surface of the test object down to a little (a few mm or less), above (inside) the rear or back wall surface; thus, the region near the rear wall surface must be excluded from inspection, i.e., the looking window for flaw echos must be closed well ahead of the occurrence of the rear wall echo.
The German printed patent application OS 24 22 439 discloses a method for automatic adjustment of the looking window, whereby echo pulses are measured and their occurrence controls the position (in time) of the looking window. This known method, however, has the disadvantage that it does not adjust to any differences in thickness of the test object. Thus, rear wall echos could be interpreted as a flaw echo, if the test object happens to be thin at the particular location. On the other hand, echos from flaws close to that rear wall may actually be missed for a thicker than normal location of the test object. These flaw signals, moreover, could influence the recognition of a reference echo, so that a wrong looking window is generated for subsequent measurements.
Another point to be considered is that one may test a sheet from both sides amounting to duplication of the equipment. Also, registration and association as well as cross checking of test data for purposes of ascertaining the true situation as to defects is quite difficult with the known equipment including, for example, compensation of the characteristics of the test heads, and referring to the respective rear wall echo.